The use of solder bumps or balls on a substrate for solder attachment to another abutting substrate by reflowing the bump or ball is known in the art. The solder bumps or balls are particularly useful with ball grid arrays (BGA's) where the balls are formed at finely spaced positions on an exterior surface of a BGA package and where the balls connect to metallization in the package. The metallization in turn is connected by vias or plated through holes and bonding wires to the contact pads of an integrated circuit chip mounted within the BGA package. The currently most used method for forming the balls is to use a silk screen to place spaced small amounts of flux into the lands of the package. "Lands" as used herein means the landing area onto which contacts are connected. This has become more and more difficult to do as the spacing between the lands becomes very small i.e., of the order of 100 microns. Silk screening can also cause flux to be laid down outside the desired small areas and the result can be that when the solder balls are placed on desired spots, a ball or balls, will float on the flux and be moved away from the desired spot as the flux melts. This can cause adjacent balls to touch and thus undesirably cause an electrical short. The function of the flux is to cause the deposited solder ball to wet the metallization and upon solidification provide a suitable metallurgical connection at the desired spot or spots. The metallization spots are normally gold plated or otherwise plated or may be bare or OFHC copper. A series of small wells or holes of the order of 0.025" (25 mils) in diameter and 0.003" (3 mils) deep, extend from the planar surface of the package downwardly to the plated spot. The goal of the silk screening is to place flux into the small holes so that each solder ball when heated will use the flux to wet and connect to a plated spot. However this goal is difficult if not impossible to obtain in production since silk-screened flux just does not repeatedly fill or even go into each and every one of the spaced holes. The flux has a tendency to just run over the holes and to pile up and when melted to run over the entire substrate surface.
U.S. Pat. No. 5,024,372 discusses other prior art such as the use of stencils and where solder paste (flux) is applied to the substrate though its stencil by using a squeegee and the use of discrete solder balls which are precisely placed on the substrate, both of which techniques having recited shortcomings. In the latter, a jig picks up by vacuum a full supply of fluidized balls, touches the held balls in a flux, places the flux-coated balls on the package and removes the jig. Flux can migrate onto the jig and the next time the jig is used to pick up balls, the jig can pick up an excess of balls in the migrated flux and then deposit the excess balls in unwanted and shorting positions on the substrate.
The `372 invention involves application of a solder resist layer on a substrate and selectively removing the resist to form wells at solder pads on the substrate. A solder paste is then applied to the substrate or directly to the substrate by a squeegee. In one aspect of the `372 invention the wells are larger in diameter than the solder pads. A solder bumped member is then connected with the bumps located in the wells. The solder paste is reflowed to wet and help bond the solder bumps and the metallized pads to effect a metallurgical connection of the higher melting temperature solder bumps to the metallized pads of the substrate.
In utilizing silk screens and stencils inherently there is an alignment problem in accurately positioning the screen or stencil correctly over the series of plated spots, controlling the amount of flux into each hole, avoiding excess flux application and avoiding the costly need for a special screen or stencil for each size of substrate, each size of hole, each hole array design and the number of holes. If 500 holes are present then the screen or stencil must have 500 apertures. Thus there has been need to accurately fill various series of holes in substrates with flux without having excess flux spill over to the edges of the substrate or a build-up of flux adjacent to the holes.